Vegetable oils contain a number of impurities such as phospholipids, fatty acids, pigments, odoriferous compounds that must be removed from the oils to make them suitable for direct human consumption. The refining of edible oils and fats can be carried out using two routes, namely chemical and physical refining. In chemical refining, the oil is treated with water followed by alkali to make it free of gums and free fatty acids. Alkali reacts with free fatty acids and produces soap which in presence of oil and water, forms considerable amount of emulsion wherein neutral oil gets occluded resulting in high oil loss. Particularly, for oils containing high free fatty acids like rice bran oil, emulsification causes higher refining losses. The soapstock produced in this process poses serious disposal problems. The physical refining method, the most viable alternative, involves processing of degummed oils where free fatty acids are removed by steam stripping along with deodorization. Consequently, oil losses are reduced, free fatty acid is distilled out with improved quality.
Physical refining is more attractive for the vegetable oils having higher contents of free fatty acids. However, practical experience with physical refining shows that it leads to desirable results only when a very good quality feed is used. Phosphorus content of less than 5 ppm is ideal. For successful operation of physical refining, efficient pretreatment steps are, therefore, of utmost importance.
The major emphasis, thus, has to be placed on pre-processing of crude oil prior to physical refining. Researchers have shifted their interests towards establishment of an efficient degumming process that reduces the phosphorus level to less than 5 ppm. Water degumming is the simplest method for removing hydratable phospholipids leaving 80 to 200 ppm of phosphorus in the oil, depending upon the type and quality of the crude oil due to the presence of non-hydratable phospholipids. A number of degumming techniques were suggested like acid degumming (Hvolvy, A., J. Am. Oil chem. Soc., 48, 971, 503), superdegumming (Ringers, H. J. and J. C. Segers, German Patent, 2609705, 1976), unidegumming (Vande Sande, et al., EP 0348004, 1989), membrane degumming etc. (Sengupta, A. K., U.S. Pat. No. 4,062,882, 1977) to remove the non-hydratable phospholipids. However, all these methods had their inherent disadvantages and could not be applied in industrial scale for all the oils irrespective of their initial quality.
Enzymatic degumming was first reported by Roehm and Lurgi and was known as Enzymax Process (Penk, G., E. et al., EP 0513709, 1992). Phospholipase A2 was used to hydrolyze the non-hydratable phospholipids to their hydratable lyso-compounds. This was applied for soybean, rapeseed and sunflower oils, after an initial water degumming step to obtain phosphorus levels upto a level of 5 ppm. The usual bleaching step after the enzymax process would lead to an oil fit for physical refining. The enzyme used was isolated from the porcine pancreas. The availability of the porcine pancreas based enzyme is limited and may be difficult to match its requirement on a commercial scale.
The Lurgi process comprises mixing the crude vegetable oil and citric acid and heating to a temperature of up to about 70° C. and then cooling to a temperature in the range of 40-45° C. and then adding NaOH and then treating with the enzyme at a temperature in the range of 40-45° C. for about 6 hours followed by centrifuging at a temperature of about 70° C. to obtain the degummed oil. The cost of this process is quite high.
Aalrust, E., et al., in their U.S. Pat. No. [5,264,367 (1993)] reported the enzymatic degumming of water degummed soybean oil containing 130 ppm of phosphorus with 1000 units of phospholipase A2 per one liter of soybean oil in presence of sodium citrate (1 g) and sodium dodecylsulfate (20 g) in 33.3 g of water. The oil was circulated about 3 times per minute by an external centrifugal pump for 3 hours followed by heating the oil to 75° C. and continuing the treatment for one further hour to obtain oil with 3 ppm of phosphorus. The main drawback of this process is employing huge amounts of sodium dodecyl sulfate. Water is also used upto 3.3%.
Yagi et al., (U.S. Pat. No. 5,532,163) also reported the enzymatic degumming using phospholipase A2 based on porcine pancreas (100 to 2000 units per kg of oil) to degum unpurified soybean oil. However, the researchers employed very huge amounts of water in the form of enzyme solution i.e., 1.5 liters of an enzyme solution per 1.5 kg of soybean oil, which is not an economically feasible process.
A few more groups reported the enzymatic degumming of vegetable oils using phospholipase A2 isolated from porcin pancreas with slight modifications to the process (Yagi, T., M. et al, JPA-2-153997, U.S. Pat. No. 5,532,163, 1996; Aalrust, E., U.S. Pat. No. 5,264,367, 1993).
It was clearly felt that to make the process commercially viable, an alternative source for the enzyme had to be found out and M/s. Novozymes came out with a microbial sources for phospholipase enzymes. Phospholipase A1 isolated from Fusarium oxysporum was utilized for the enzymatic degumming of vegetable oils (K. Clausen, Eur. J. Lipid Sci. Technol. 103, 2001, p. 333-340). In the reported enzymatic processes mainly soybean, rapeseed and sunflower oils were subjected to an initial step of conventional water degumming. The water degummed oil was then mixed with 0.1 to 0.15% of citric acid at a temperature of 70-75° C. using a high shear mixer and cooled to 40-60° C. and then 0.03 to 0.05% of sodium hydroxide was added to it and mixed thoroughly. In the next step, 100 to 800 LEU of Lecitase-Novo and more preferably 375 units per kg of oil was mixed using a high shear mixer and then kept under stirring for 5-6 hours at 40° C. The oil was heated to 70° C. and centrifuged to get enzymatically degummed oil. The major disadvantages of the reported processes are addition of citric acid, sodium hydroxide and enzyme solution in water in a series of operations maintaining specific pH range and higher reaction times. The enzymatic degumming was also not extended to rice bran oil herein.
Loeffler et al in their U.S. Pat. No. (6,001,640, 1999) reported the enzymatic degumming of wet-degummed soybean oil with a residual phosphorus content of 190 ppm at 40° C. in presence of water (a 5% on the basis of oil) and citric acid (1%) using phospholipase isolated from Aspergillus niger. The low phosphorus content of <10 ppm was achieved in 6 hours. The process requires higher percent of water, citric acid and also longer hours of degumming period.
EP-A 0622 446 describes an enzymatic process for degumming of oils, which comprises several processing steps. After treatment with phospholipase the enzyme solution was centrifuged off, the remaining oil washed with water at a pH of 3-6, and finally treated with fuller's earth. During the enzymatic treatment and the washing step, large amounts of water was used specifically 30-200 weight % with reference to the oil used.